Male-to-female (M-*F) transmission of HIV is a major force driving expansion of the global AIDS epidemic, Our overall objective is to assess the potentially important role played by select viral and host factors in HIV transmission biology. In Specific Aim 1, HIV transmission pairs will be used to test the hypothesis that the envelope proteins from viruses successfully transmitted to women differ from non-transmitted viruses by their fusogenic properties and/or target cell selectivity. Envelopes from transmitted viruses share common features including compact V1-V4 regions, decreased numbers of glycosylation sites, and a greater sensitivity to neutralizing antibodies. These viral envelopes may be imbued with special properties that favor virion fusion to target cells within the female reproductive mucosa. In Specific Aim 2, biological features of recently identified fibrillar peptides in semen that can enhance HIV infection 10-100,000-fold will be tested. These peptides, termed semen enhancer of HIV infection (SEVI), are produced by endoproteolytic cleavage of prostatic acid phosphatase, a highly abundant semen protein. They enhance HIV infection by sharply increasing virion attachment to target cells. Our studies will test the hypothesis that SEVI propels M->F transmission of HIV and further that SEVI antagonists may impair HIV infection of women. Our studies specifically seek to: 1) identify the cellular protease(s) responsible for the generation of SEVI, 2) test whether SEVI enhances virion transcytosis, frans-infection, and cell-cell infection, 3) explore whether SEVI production varies among males and whether HIV infection alters SEVI levels, and 4) investigate novel approaches for inhibiting the action of SEVI. Effective SEVI antagonists could find application in future multi-component microbicides designed to prevent HIV transmission to women. This project will also interact extensively with Projects 2 and 3 of this P01 application. Viruses containing primary transmitted envelopes will be provided for use in these projects. Fusion assays will be performed collaboratively to monitor effects of progestin contraceptives, menopause, and microbicides on HIV target cells within female reproductive mucosa. Semen, SEVI, and SEVI antagonists will also be profiled for potential harm effects on endometrial and cervical mucosa. Together, the multiple lines of investigation described in Project 1 promise to expand our understanding of the molecular basis for M-F transmission of HIV.